Abstract Engrailed (En) is a transcription factor; a protein that binds to DNA and that switches on or off other genes. It was first discovered in Drosophila but later found to be present in all animals, where it plays an important role in controlling the development of neurons, or nerve cells. The long-term goal of this research is to find out how En controls the way that nerve cells connect to each other (form synapses) within the brain, with a particular focus on identifying and characterizing the network of genes and molecules that it regulates. This proposal uses the Drosophila auditory neuron ? to ? giant fiber (GF) synapse as a model system with which to investigate the role of En in the control of synaptic connections. The first specific aim will be to use electrophysiology and anatomical staining techniques to measure how auditory neuron action potentials and output synapses to the GF change as the animal ages, and to determine whether there are sex-specific differences. The second specific aim will be to study the effects of changing the amounts of En present in the neurons in the adult animal. As in mammalian neurons, En expression in Drosophila auditory neurons persists through adult life, but its functions during this period are not understood. It could perhaps maintain their electrical properties or the patterns of their synaptic connections. To test this, genetic methods will be used to add or take away En from the sensory neurons at different times. This work is relevant to public health because human En has been linked to several brain disorders, such as Parkinson?s disease and autism. Drosophila models are particularly useful for the discovery of molecular pathways that are directly relevant to humans, because most of these pathways have been conserved during evolution. All animals have En protein, so it is very likely that any molecules that are regulated by it during the process of synapse formation in Drosophila have their counterparts in humans, playing similar roles in brain development.